Underground caverns of a large scale built at very deep places of the kind mentioned above are extremely big caverns having a depth of about 100 meters and inside dimensions of about 100 meters, and are utilized for underground power stations and natural resources storing depots, etc. Such underground caverns are generally built in hard rock beds.
However, with the development of high-degree utilization of underground space of late years, there are strong demands for building such caverns of a large scale at very deep places in soft grounds even in urban communities.
In the case of building such caverns in soft grounds, two methods have so far been used. One method is to build earth retaining walls "a" so as to surround a required space and then excavate earth and sand within the space, as shown in FIG. 1; this is called, "Continuous Wall Building Method" (Open Excavating Method). Another method is to excavate a tunnel "b" of a large diameter during which lock bolts "c" are driven from the inner surface of the tunnel into the ground simultaneously with injection of a grout, and then form a concrete wall "d" on the inner surface by spraying cement mortor; this is called, NATM method.
Of the above-mentioned prior art methods, the former is disadvantageous in that, if a cavern or a tunnel is to be built at a very deep place, then the thickness of the earth retaining walls "a" must be increased, resulting in an increase in the earth retaining wall building cost. At the same time, the latter is a method which has been developed of late years for use in building tunnels in mountainous districts. In the case of the latter method, if the scale of the tunnel to be built is large, then the thickness of the rock beds to be reinforced becomes large, and reinforcing work is effected while excavation of the tunnel is being made. Therefore, there occur losses in time due to changes in arrangement of operations and uncertainty in ensuring safety during excavation.